1. Field of the Invention
The present invention relates to a sheet feeding apparatus used with an image forming system such as a copying machine, laser beam printer and the like.
2. Related Background Art
In the past, as shown in FIG. 21, there has been proposed an image forming system wherein sheets can be automatically supplied from a sheet supply cassette 6 and also can be supplied manually from a manual sheet supply tray 22. In such a conventional image forming system, when the sheet P is supplied automatically, the sheet fed from the sheet supply cassette 6 by a sheet supply roller 10 is fed along a guide G1 by means of a feed roller 11 to reach a photosensitive drum 15 through a space between a lower guide G2 and an upper guide G3.
On the other hand, when the sheet is supplied by the manual sheet insertion, the sheet P set on the manual sheet supply tray 22 is fed to the photosensitive drum 15 through the space between the lower and upper guides G2, G3. Incidentally, the reference numerals 13a, 13b, 13c denote sub-rollers which cooperate with the feed roller to pinch the sheet therebetween for feeding the sheet.
A sensor 20 is disposed at a junction between an automatic sheet feeding path and a manual sheet feeding path. In case of the automatic sheet supply, the sensor 20 serves to register or synchronize the sheet supplied from the sheet supply cassette 6 and a copy timing of the photosensitive drum 15. In case of the manual sheet insertion, the sensor 20 serves to switch an automatic sheet supply mode to a manual sheet insertion mode when the sensor detects the manually inserted sheet P, and to synchronize the manually inserted sheet P with the copy timing of the photosensitive drum 15.
In the above-mentioned conventional image forming system, both in the case of the automatic sheet supply and in the case of the manual sheet insertion, when the sheet P is detected by the sensor 20, the feed roller 11 is stopped to interrupt the feeding of the sheet temporarily. Thereafter, when the copy timing is established, the feed roller is rotated again to send the sheet to the photosensitive drum 15. For example, in a laser beam printer, a data signal sent from a host computer is changed to an image formation signal by a signal processing means, and a laser scanner unit emits a laser beam on the basis of the image formation signal to form an electrostatic latent image on a photosensitive drum. The electrostatic latent image is developed as a toner image, which is then transferred onto a sheet. Now, the sheet P supplied from the sheet supply cassette 6 and the manually inserted sheet P from the manual sheet supply tray 22 is stopped temporality when they are detected by the sensor 20, and are waiting at the stopped position, until the data signal is changed to the signal formation signal by the image processing means. Thus, when a distance between the waiting position and a transfer position was long, it took a long time to feed the sheet from the waiting position to the transfer position, thereby extending the print time. Particularly, in a copying machine or a printer, it has been desired to shorten to the image formation time. Therefore, it is important to reduce the sheet feeding time.